Dynamical transmission electron microscopy microstructure associated with electron-phonon interaction in high-damping Cu‒Mn alloys

Abstract

Abstract The transformation behaviour of Cu-Mn-based high damping alloys aged within a miscibility gap has been studied intensively using transmission electron microscopy (TEM), with particular attention to fine-scale dynamical movements in segregated microstructures, termed 'flickering'. As ageing proceeds, features appear in sequence in TEM images: mottled, tweed, and V-shaped images are observed in this order. The image contrast of these features, as well as streaks in diffraction patterns, and their extinctions, have been consistently explained by assuming a growth of the static displacement field of the {110}⟨110⟩ mode in the fcc lattice, in agreement with other 'tweed-forming' materials. In addition, extra diffuse intensities due to {110}* relwalls in diffraction patterns are observed, which are considered to arise from a dynamic displacement wave of the (110}⟨110⟩ mode. Distinctive 'flickering' movements are observed at a stage of ageing corresponding to a transition from the late stage of "tweed" to the "V-shaped" stage, i.e., in the microstructure of highest-damping capability. The flickering movements are interpreted as a direct manifestation of the fcc⇆fct transformation, with a rotation of the fct tetragonal axis taking place in Mn-enriched confined regions. This is driven by a ⟨110⟩⟨110⟩ phonon coupling and decoupling process under the inelastic interaction of the phonons with the accelerated electrons. A similar dynamical effect known as "streaming" is separately observed, which is considered to be another manifestation of the electron-phonon interaction taking place in the Cu-rich matrix in this system.